Ethylene, propylene and butadiene are important chemical raw materials, and are generally obtained by pyrolysis or steam cracking of naphtha. At present, the main source of propylene includes propylene co-produced with ethylene and propylene as a refinery byproduct. The main source of butadiene is by further processing the C4 byproducts produced in an ethylene cracking process. However, these manners of production can hardly meet the increasing demand for propylene and butadiene in China. It has been highly desirable to develop a process for preparing propylene and C4 olefin at high selectivity.
As such, the researchers in Dalian Institute of Chemical Physics have developed a DMTO technology which uses an aqueous solution of methanol as raw material in a reaction in the presence of a catalyst of SAPO-34 molecular sieve. The main products are ethylene and propylene, but the selectivity to C4 olefin is very low, more particularly, only less than 10%.
According to the fixed-bed technology for preparing olefin from methanol developed by Lurgi Co., Germany (WO2004/018089), a ZSM-5 molecular sieve purchased from Süd-Chemie Co. is used as a catalyst to carry out the reaction for preparing olefin from methanol in a fixed-bed reactor, and the resultant selectivity to propylene is 35-40%. CN102531823A, CN101172918B, CN101279280B, CN101402049A, CN102211971A disclose respectively processes for preparing propylene from methanol in fixed-bed reaction systems and methods for preparing catalysts. More particularly, these processes mainly improve the single-pass selectivity to propylene by modifying a ZSM-5 molecular sieve which acts as an active component. However, most of these patent applications focus on improving the single-pass selectivity to propylene, but neglect the selectivity to C4 olefin. In the Lurgi process which has already been put into real industrial use, C4 and C5 olefins are recycled to the MTP reactor for continuing reaction to produce propylene.
In a research report, there is reported a technique for preparing a catalyst used to prepare olefin from methanol in a fluid bed, wherein a ZSM-5 molecular sieve is used as the active component, and modifiers such as rare earth modifier, alkaline modifier and like are added to improve the activity of the molecular sieve. An aqueous solution of methanol is used as a raw material, and the catalyst is used to carry out a reaction where the resultant propylene selectivity is up to 55%, whereas the C4 selectivity is not high, only 25%.
As can be seen, the previous research on the process of preparing olefin from methanol mainly concentrates on improving the single-pass selectivity to propylene and the propylene/ethylene ratio in the products, whereas the selectivity to C4 products is not a focus. The practice of recycling the C4 products to the reactor for continuing reaction and producing propylene in the MTP process complicates the process, and it's very difficult to guarantee the selectivity to propylene. On the other hand, there is a high demand for butadiene in the domestic market, and thus the market price of butadiene has been high for a long time. If a methanol-to-olefin reaction can be enabled to have high C4 olefin selectivity while the high selectivity to the propylene product is ensured, and butadiene is produced by separating butene from the C4 olefin products and dehydrogenating the butene, additional and high economic profit will be obtained.